Spacecraft require lightweight, high efficiency, long lasting, high heat/low energy thermal electrical elements for both manned and unmanned vehicles. Traditionally these requirements have been met using wire-type heating elements. However, traditional wire-type heating elements require coatings for oxidation and corrosion resistance. Moreover, such heating elements are expensive to fabricate, bulky, and consume large amounts of power during operation. Devices of this type are also fragile and have relatively short lifetimes.
The present invention is directed to a process for fabricating solid state, reliable thermal heating elements having (1) a low thermal mass, (2) extremely high corrosion resistances, (3) uniform heat output over large areas, (4) excellent thermal heat transfer characteristics, and (5) maximum electric power efficiency with minimum weight using a novel low cost process. The solid state devices produced by the present invention satisfy all the above-stated heating element requirements and can be applied in tiny microelectronic heating elements, immersion heaters, and liquid and solid heating units, including very-large-area radiant heating panels.
It is well known that tungsten has the highest melting point and lowest vapor pressure of all metals. Tungsten is obtained commercially by reducing tungsten oxide with hydrogen or carbon. Pure tungsten is steel-gray to tin-white in color. The impure metal is brittle and can be worked only with difficulty. Pure tungsten can be cut with a hacksaw and can further be forged, spun, drawn, and extruded. It is the extrusion property combined with its high melting point that facilitates the use of tungsten in light filaments. Unfortunately, tungsten oxidizes at about 450.degree. C. Therefore, a high vacuum is required to assure long incandescent filament lifetime. Even tiny quantities of water vapor or oxygen in the bulb greatly reduce filament lifetime.
An added problem attendant to the use of tungsten is the requirement for additives like potassium, silicon, and aluminum in order to allow proper swaging and wire-drawing. Moreover, these additives in tungsten wire are detrimental when tungsten filaments are used in evaporation processes involving other materials, because some of the impurities get into the pure metal being evaporated. Also, outgassing of these materials can affect the lifetime of tungsten light bulbs. A final problem associated with tungsten is the migration of impurities along grain boundries in tungsten filaments, which eventually causes cracking of the wire.
Traditionally, incandescent lamps have been fabricated by the following procedure. First, tungsten acid (W03:H20) is added to potassium silicate in an amount such that the potassium content in the KCl is 0.40%, the silicon content in SiO.sub.2 is 0.30%, and Ga(NO.sub.3)3 is added in an amount such that the Ga content expressed in GA203 is 0.05%. This paste is then dried, dehydrated at 300.degree. C., and reduced in a hydrogen furnace at a temperature of 850.degree. C. or higher. The metal powder prepared by this procedure is processed by high pressure extrusion and sintered into a rod. At this stage of the process, crystals are visible on the surface of the rod. Subsequently, the rod is processed through wire drawing dies using swaging, or other mechanical wire drawing techniques, in order to produce incandescent wire for light bulbs. The specific process steps used, the impurity content, and the size of the tungsten crystals actually formed are critical factors in achieving the mechanical shock resistance, wire break resistance, and incandescent properties required for luminescense. The drawn wire is next spiraled into a filament to increase the potential lumens per unit area. The ends of the filament are then welded to conductive support wires which are then attached to a socket support. This socket support with filament assembly is then inserted into the bulb, sealed, and subsequently, the air in the bulb is removed using an indexing mechanical vacuum pumping assembly to assure formation of a high vacuum within the bulb so as to prevent oxidation of the tungsten filament.
A second method for forming a tungsten incandescent filament is described in U.S. Pat. No. 3,811,936 whereby a drawn tungsten filament wire is increased in cross-sectional area by means of hydrogen reduction of tungsten hexafluoride onto the hot filament. By this means, purer tungsten filament wires can be fabricated.
Heating elements comprising electrical resistance wire supported by ceramic materials are also known in the art and are described, for example, by Pauls in U.S. Pat. No. 3,436,540. Heating elements comprising electric resistance wire sealed into or supported by ceramic insulators, such as nichrome wire tightly sealed into an alumina/silica ceramics, is described by Erickson in U.S. Pat. No. 4,596,922. Other forms of heating elements use metal foil etched into a serpentine pattern whereby application of electric power allows heating of the metal. Devices of this type are usually supported back and front by use of insulating panels. A heating element of the foil type is described, for example, by Furtek in U.S. Pat. No. 4,659,906. Gyuris describes use of a heavy metal foil etched into a grid-like pattern to allow electrical resistance heating of electric irons or other electrical applicances in U.S. Pat. No. 2,553,762.
In the semiconductor industry large tube-type heating elements, sometimes a foot in diameter and several feet long are also used to heat silicon wafers. Because of the heavy wire required for the high temperatures of these operations, a considerable mass of insulating material is required. Relatively expensive stepdown transformers or silicon controlled rectifiers are required to provide the high amperages required to operate such heavy resistance wire as a consequence of the fact that a good deal of the energy produced by the electrical elements is wasted through dissipation into the insulating material.
In summary, the present state of the art has relied on wires, etched metal foils, and resistive metal bars supported by heavy refractory materials to accomplish resistance heating. Difficulty has been experienced in the manufacture of such elements because a large number of closely-spaced wire or foil elements must be held in close proximity by use of insulating materials that are hard to fabricate and have a non-matching coefficient of expansion. To hold such wires and foils in alignment a considerable amount of refractory insulator is required, which in turn, results in considerable wasted heat and electrical energy. Consequently, it has thus far been extemely difficult to provide heating elements having sufficient rigidity and mechanical strength to prevent buckling and contracting of adjacent resistance wire segments while minimizing wasted heat.
While heating element manufacture and use has heretofore encountered the above-discussed problems associated with prior art fabrication methods, materials combining Silicon/Quartz/Tungsten provide an ideal system for solid state heating elements. Initially, no metallic material matches the coefficient of thermal expansion of silicon better than tungsten. This property is extremely important for the thermal cycling required by a solid state heater. The melting points of these components and the eutectics formed are also very important. The melting point of silicon is very high (1410 degrees C.), and tungsten is almost 21/2 times higher (3410 degrees C.). Further, quartz, which is used to provide the corrosion resistant covering over the tungsten, melts at a temperature higher than silicon (1665 degrees C.). The eutectic temperature of alloy formation of tungsten/silicon occurs at 1400.degree. C. The vapor pressures of all three materials and their combinations are extremely low at elevated temperatures. The electrical resistivity of tungsten provides excellent properties as a heater material, having about one-half the resistivity of platinum and substantially less resistivity than nickel. The thermal conductivity of silicon is good, better than that of nickel, and about equal to tungsten.
All of the above considerations are important for a successful solid state heater. Because of the applicability of the Silicon/Tungsten/Quartz system to microelectronic applications, the metallurgical system has been well-characterized and researched. Thus, most of the basic materials development has already been performed. The results of recent studies have predicted that a tungsten-silicon metalization system will provide much more reliability than an aluminum-silicon system, normally used for IC manufacture, because of the much more optimum thermal expansion coefficient match of tungsten/silicon (i.e., 4.5/3) compared to the much greater mismatch of aluminum/silicon (i.e., 25/3).
Another major advantage of the tungsten/silicon system is that "spiking" and electromigration of the metallic element does not occur even at elevated temperatures in this system. These phenomena have plagued IC and rectifier manufacturers who use the aluminum/silicon system for years. The problem causes eventual shorting of IC's because of electrical current induced electromigration of the aluminum. To attempt overcoming the problem aluminum interconnect metal has been alloyed with copper and silicon making it necessary to use expensive sputtering processes at IC manufacture.
In summary it would be highly desirable if incalescent tungsten filament heating elements could be formed in association with high purity silicon material. This would eliminate expensive wire drawing operations, and the requirement for refactory materials supporting the wires as is required by the prior art. Further, it would be desirable to accomplish this as a solid state device by inherently low cost process steps. This is the main goal of the present invention herein described.
A second objective of the present invention is to provide a method for making a resistive heating element in which a desired complex heating electrode pattern is easily fabricated.
A further objective of the invention is to provide a method for making resistive heating elements wherein wire and foil, with their attendant attachment and alignment problems, are eliminated.
It is another objective of the present invention to provide improved thermal contact of the heating electrodes to the substrate support material.
It is still yet another objective of the present invention to allow formation of resistive electrodes in intimate contact with substrate materials of many differing types [not only refractories but also metals].
Another objective of the invention is to allow formation of resistive electrodes to objects of complex topology.
Another objective of the invention is to allow formation of resistive heating elements particularly resistant to corrosion.
Finally, it is an objective of the present invention to Provide new lightweight electrical heating elements having very low thermal mass such that fast thermal rise and cooling times are possible.